Mozzarella cheese production has grown considerably in recent years, especially in the USA, where current annual production, at circa 938,000 tonnes, amounts to circa 29% of total U.S. cheese manufactured (McMahon et al., 1993; Residuary Milk Marketing Board, 1995; Personal communication, 1996--Michael Hickey, Member of IDF Group D31). This markedly exceeds that of Italy (i.e. 150,000 tonnes), the country of origin and world's second largest producer. The growth rate has been slower elsewhere but is now escalating, with significant expansion in Ireland, Denmark and New Zealand. The impetus for the dynamic growth of Mozzarella consumption has been the growing popularity of pizza pie, i.e. baked dough pie covered with cheese, tomato puree and other adjuncts such as salami and mushroom. The main type of Mozzarella used in this application is low moisture (45-52%), (USDA, 1976; Office of the Federal Register, 1986), the traditional Mozzarella (.about.57-60% moisture) being consumed mainly as a table cheese or as an accompaniment in salads.
The shredability of low moisture Mozzarella and its capacity to melt, flow, stretch and possess chewiness on cooking/baking make it ideal for use in pizza pies. However, its ability to stretch and form long strings when extended makes low moisture Mozzarella unique in pizza applications. Other cheeses, such as Cheddar and Provolone also have the ability to melt and flow on cooking/baking. However, these cheeses which have a relatively high level of proteolysis when mature, exhibit inferior stretch properties compared to low moisture Mozzarella. Owing to the widespread use of cheese in cooking applications, numerous studies have been undertaken vis-a-vis the development of methodology for assessing the melt, flow and consistency characteristics of cooked cheese (Arnott et al., 1957; Kosikowski, 1982; Rayan et al., 1980; Hokes et al., 1982; Park et al., 1984; Masi 1989; Kindstedt et al., 1989a, b). In contrast few investigations have dealt with the measurement of the capacity of coked molten cheese to stretch when extended. At commercial level this property is largely assessed by lifting the cooked cheese with a fork. While this method has merit in that it simulates consumer behaviour, it is very subjective as a quality control tool, as the stretchability depends on the depth to which the fork is embedded in the molten cheese mass and the rate at which it is lifted. Hence, objective methods for the determination of stretchability have appeared recently in the scientific literature (Addeo & Masi, 1992; Cavella et al., 1992; Ak et al., 1993; Apostolopoulos, 1994; Pagliarini & Beatrice, 1994; Ak & Gunasekaran, 1995). Some of these methods are based on uniaxial extension of heated cheese samples (e.g. slices, cylinders) of given dimensions using a Universal Instron-type instrument or load-cell (Ak et al., 1993; Pagliarini & Beatrice, 1994; Ak & Gunasekaran, 1995). The resulting force-displacement curves were used to determine the force required to achieve a certain displacement (i.e. stretch), the force and displacement at fracture and/or the elastic modulus and viscosity of the extended sample. The methods of Addeo & Masi, 1992 and Cavella et al. (1992), involved measurement of the force required to break a string (.about.1 mm thick) of melted cheese, extruded using a piston type capillary rheometer, and the percentage elongation of the string at fracture. While these methods are objective in their approach, their complexity and the operator skill required makes them somewhat unsuitable for routine rapid evaluation of stretchability. Moreover, the methods employ conditions of cheese melting (e.g. in oil bath at temperatures of 10-40.degree. C.) and extension which do not simulate very closely those used in commercial pizza preparation and consumption.
A more empirical operator-friendly tensile test method, employing heating and stretching conditions closer to those used in practice was recently reported by Apostolopoulos (1994). Shredded cheese, at a certain loading, was placed on a pre-cut circular pizza base before heating in a microwave oven, after which the pizza was stretched. To allow a vertical stretching motion the circular cheese base was pre-cut to fit over a matching perspex disc which had a fixed annular ring and a moveable circular core with a central cross-head attachment. After loading the shredded cheese, the entire assembly (including supporting "Perspex" .TM. disc) was placed in a microwave oven. On removal of the cooked pizza pie the core was attached to the cross-head of the tensile testing machine and raised at constant velocity (normally 1.5 m/min) until fracture of the cheese strings. The extensibility of the cheese, defined as the distance of travel of the cross-head until all the cheese strings failed, was used as a measure of stretchability. Details on the effects of cheese loading, the holding time of the cooked pizza before stretching and/or age and variety of cheese are not given.
WO 90/08306 discloses an apparatus and method for performing a tensile test on natural or man-made fibers such as cotton or polyester. The apparatus includes two clamps between which each end of a fiber can be clamped. One clamp is stationary while the other is associated with a tensioning device, which includes a stepping motor which is connected to the clamp by a connecting member so that by appropriate actuating of the motor, the clamp can be moved away from the stationary clamp to tension the fiber. The device monitors and measures tension force and elongation of the fiber.